1. Field of the Invention
This invention relates to microwave buffer circuits, and in particular to monolithic microwave buffer amplifiers having high input impedances.
2. Description of Related Art
High-input-impedance microwave buffer amplifiers are particularly useful for coupling cascaded microwave circuits and/or components. When utilized in this manner, such a buffer amplifier couples an output of a first microwave circuit or a microwave component with an input of a second microwave circuit or component to prevent the input from loading down the output.
In monolithic microwave buffer amplifiers, and particularly in the case of field-effect transistor (FET) or bipolar transistor stages, the influence of the transistor input capacitance on the buffer amplifier input impedance becomes increasingly dominant over that of the transistor input resistance as the operating frequency is increased above the 3 dB rolloff frequency f.sub.3dB. As used in this patent application, the term 3 dB rolloff frequency means that frequency at which the gain drops more than 3 dB below the nominal DC open-loop gain A of the amplifier. An example of this gain dropoff is illustrated in FIG. 1. Typically, the input impedance Z.sub.IN is equivalent to an input resistance R.sub.i electrically connected in parallel with an input capacitance C.sub.i. As the operating frequency of the buffer amplifier is increased above f.sub.3dB, the capacitive reactance 1/2.pi.fC.sub.i decreases relative to the parallel resistance R.sub.i resulting in a comparable decrease in the magnitude of the input impedance. An example of this decrease is also illustrated in FIG. 1. Note from the example that the magnitude of the input impedance Z.sub.IN of such a buffer amplifier at the microwave frequency of 1 GHz can be a very low value, such as 2 ohms. This can severely load the output of a circuit or a microwave component coupled to the input. This input characteristic of monolithic microwave buffer amplifiers typically limits their usefulness to situations where the microwave circuits and/or components are operated at frequencies below the 3 dB rolloff frequency of the buffer amplifiers.